posted by jane c .
mass of dry crucible=34.69g
mass of crucible with magnesium=34.68g
mass of crucible with magnesium oxide =34.97g
we did a lab and igot the mass for magnesium to be 0.17g and the mass for magnesium oxide to be 0.28g.
how can i find the moles of magnesium and the number of molecules of magnesium oxide
You may have made a typo but mass Mg from your data is -0.01 which is impossible.
To answer your question,
mols Mg = mass Mg/atomic mass mg.
# molecules MgO (I wonder if you don't want mols of this, too) = (mass MgO/molar mass MgO)* 6.022E23 = ?